1. Field of the Invention
The present invention relates to a relay circuit and a method of testing a relay circuit. More particularly, the present invention relates to a relay circuit for controlling a load in response to a control device, where the relay circuit has a test switch for testing the relay circuit while isolating the control device.
2. Discussion of the Related Art
FIG. 1 shows a conventional relay circuit 10 connected to a load device 60 and a control device 70. Conventional relay circuit 10 includes a transformer 20 having four terminals, a relay switch 30, and a resistor 40. A coil 35 of relay switch 30 and resistor 40 are connected in parallel with one common node connected to a first terminal 21 of transformer 20, and the other node connected to a terminal 51. The conventional relay circuit shown in FIG. 1 is provided on a circuit board and terminals 22, 51, 52, and 53 are also provided on the circuit board to allow external components to be connected. For example, a load device 60, such as a hot water heater, heating/air conditioning equipment, circulator, oil burner, gas valve, or any other line voltage load, may be connected to load terminals 52 and 53, while a control device 70 such as a thermostat may be connected to terminals 22 and 51. Terminals 23, 24, 54, and 55 are provided on the circuit board to provide connection to a power supply, such as a commercial 120 V AC power supply. Terminal 56 may optionally be provided to connect a second load device that operates whenever the first load device 60 is not operating.
In normal operation, control device 70 will provide a closed circuit across terminals 22 and 51 thereby causing relay coil 35 to close relay switch 30 across terminals 31 and 32 and causing power to be delivered to load device 60 in order to cause load device 60 to turn on. Similarly, control device 70 will provide an open circuit between terminals 22 and 51 thereby causing relay switch 30 to interrupt the power supplied to load device 60 in order to cause load device 60 to turn off.
When a system failure is detected, one must be able to individually test the components of the system. Because relay circuits, such as that shown in FIG. 1, are prone to failure, one must be able to test the operability of the relay circuit to determine whether relay circuit 10 has failed or control device 70 has failed. Typically, one would test the operability of relay circuit 10 by shorting across terminals 22 and 51. If load device 60 turns on when terminals 22 and 51 are short-circuited, one may conclude that relay circuit 10 is operational and the failure is caused by control device 70. On the other hand, if load device 60 does not turn on, one may conclude that the failure is caused by relay circuit 10 or load device 60.
Because control devices, particularly electronic thermostats, are prone to damage resulting from backfeeding, the above described method of testing the relay circuit subjects the control device to damage.